The present invention relates to an automatic document feeding apparatus, and more particularly to an improvement in an apparatus for automatically conveying a document to a predetermined reading position in a recording device or an image reading device of an electrophotographic copier.
An automatic document feeder is generally provided on a platen glass in an image exposure section of a copier. In an optical system movement type copier to which the automatic document feeder is provided, documents are conveyed as follows: documents are sent out from a document stack tray (stack section) by the automatic document feeder one by one; conveyed onto the platen glass by a conveyance belt; and after the documents have been exposed by an exposure lamp provided in the copier body, they are discharged onto a document discharge tray.
In a conventional automatic document feeder, the document feeding operation is carried out in the following manner:
after a paper discharge sensor has detected that a document on the platen has been discharged, the following document is sent out from the stack section onto the platen glass, and stopped at a predetermined position. Therefore, it takes a long period of time to replace a document placed on the platen. Accordingly, a large amount of time is required for a copying operation, and the productivity is low.
In the case where an automatic document feeder is used, the copying productivity can be increased to 100% when a document is replaced within a period of time in which the optical scanning system is returned. In order to replace the document within the period of time in which the optical scanning system is returned, it is necessary to increase the document conveyance speed when the document is conveyed from a double feeding prevention device of the document stack section to a document stop position on the platen glass.
However, in order to convey the document at high speed, various problems are caused, such as an increase in the motor dimensions, an increase in electrical power consumption, noises, abrasion of the conveyance belt, damage of the document, and deterioration of the document separation property.
In the case where a two-sided document is copied, the two-sided document is conveyed onto a platen glass. After the first surface of the document has been exposed, it is reversed by a document reversal conveyance means, and conveyed again onto the platen glass, and the second surface of the document is exposed. After that, the document is reversed again by the document reversal conveyance means, and then the document is discharged outside of the apparatus under the condition that the discharged documents are collated. In the above automatic document feeding apparatus for feeding two-sided documents, an interval between the trailing end of the first document and the leading end of the second document is long. Therefore, the second document travels over a long distance in the case where the document is replaced or reversed. Accordingly, a period of time required for replacing a document is disadvantageously increased.
In order to overcome the disadvantages described above, the present applicant has proposed the following automatic document feeding apparatus disclosed in Japanese Patent Publication Open to Public Inspection No. 330736/1991 (corresponding to Japanese Patent Publication No. 5-165284). While the second surface of the previous document is being read (that is, while the conveyance belt is stopped), the next document is sent from a document tray. The document is made to wait for the next operation at a position close to the upstream of the conveyance belt. When the previous document is discharged after the reading operation of its second surface has been completed, the next document that has been waiting for the following operation is conveyed to the reading position.
However, in the apparatus constituted in the manner described above, an interval between the trailing end of the previous document and the leading end of the next document is restricted by a positional relation between the reading section on the platen glass and the upstream end portion of the conveyance belt (that is, the interval is restricted by a distance from the upstream end portion of the conveyance belt to the reading position). Therefore, it is impossible to reduce the interval between the trailing end of the previous document and the leading end of the next document for the purpose of shortening the document replacing time.
The present invention has been achieved in consideration of the aforementioned problems. It is an object of the present invention to reduce a period of time required for replacing a document in order to increase the copying speed when the next document is conveyed to a position as close to the previous document as possible while the second surface of the previous document is being read, without being restricted by the conveyance distance of the conveyance belt. Also, it is another object of the present invention to stabilize a paper discharging operation while the reduction of the document replacing time is realized.